Ecosystems worldwide are receiving increasing amounts of reactive nitrogen (N) through anthropogenic activities. Bacteria play critical roles in ecosystem processes and identifying how anthropogenic N impacts bacterial communities may elucidate how critical microbially-mediated ecosystem functions are altered by N additions. Our previous work on experimental N gradients at Cedar Creek, MN and Kellogg Biological Station, MI demonstrated that N fertilization consistently impacts both the phylogenetic and taxonomic structure of soil bacterial community structure in a predictable manner regardless of ecosystem type. These results suggest that bacterial communities across N fertility gradients are structured more by either nitrogen and/or soil carbon availability, rather than by shifts in the plant community or soil pH indirectly associated with the elevated nitrogen inputs. Still, field N additions can have both direct and indirect effects on microbial communities and previous work was unable to fully decouple N and C effects with this study. In an effort to identify the bacterial groups that respond directly to N additions we preformed an in-lab follow up experiment. Four concentrations (10, 50, 200, 500 ug N) of six N types (NH₄NO₃, Urea, KNO₃, NH₄Cl, (NH₄)₂SO₄, Ca(NO₃)₂) were added to three distinct soils. Soil respiration (CO₂) was measured and analyzed using an infrared gas analyzer (IRGA) for 45 days.
Results/Conclusions

Across all soil types we consistently observed a significant decrease in soil respiration of approximately 60%, indicating that N additions functionally impact the soil bacterial community. Using high-throughput pyrosequencing we are able to identify which bacterial groups respond directly to in-lab N additions. By comparing our in-lab community composition and functional changes to our field results we build a predictive understanding of how N additions impact the structure and function of soil microbial communities.